Glasses with very high refractive index

ABSTRACT

The invention concerns glasses that have a refractive index of more than 1.880, an Abbe number of more than or equal to 30.4, and a density of more than 4.1, and having the following chemical composition, expressed in wt. %, based on the oxides: SiO 2  : 6-9, B 2  O 3  : 15-19, with SiO 2  +B 2  O 3  &lt;26, Nb 2  O 5  : 12 to &lt;20.5, ZrO 2  : 8-10, TiO 2  : 7-11, La 2  O 3  : 23-32, with ZrO 2  +TiO 2  +La 2  O 3  : ≧44, CaO: 8 to &lt;13, SrO: 0-2, BaO: 0-2, MgO: 0-2, ZnO: 0-5, with CaO+SrO+BaO+MgO+ZnO: ≧9, Y 2  O 3  : 0-5, WO 3  : 0-5, Li 2  O+Na 2  O+K 2  O: 0-2, others 0-1.

FIELD OF THE INVENTION BACKGROUND OF THE INVENTION

The present invention relates to glasses having a refractive index ofmore than 1.880, an Abbe number of more than or equal to 30.4, and adensity of less than 4.1.

Such glasses are of great interest for ophthalmic applications,particularly in the case of large corrective-vision levels: the highrefractive index allows limiting the thickness of the glasses. Themoderate density and the relatively high Abbe number also contribute tothe comfort of the wearer.

Moreover, it is advantageous to have glasses that are easy tomanufacture and that have a low manufacturing cost. For that purpose, alow cost of raw materials and a low tendency toward devitrification aredesired elements.

PRIOR ART

Glasses presenting optical properties and a similar density to thosedescribed above are already known, but they are not satisfactory insofaras the cost of manufacture and the resistance to devitrification isconcerned.

Thus, European Patent No. B-570,687 describes glasses having thefollowing composition, expressed in wt % based on the oxides:

    ______________________________________                                        SiO.sub.2           5-8                                                         B.sub.2 O.sub.3 15-21                                                         ZrO.sub.2  3-10                                                               TiO.sub.2  7-17                                                               Nb.sub.2 O.sub.5 20.5-26                                                      with ZrO.sub.2 + Nb.sub.2 O.sub.5 29-45                                       La.sub.2 O.sub.3 19-32                                                        Y.sub.2 O.sub.3 0-9                                                           CaO  8-16                                                                     SrO 0-5                                                                       BaO 0-5                                                                       MgO 0-5                                                                       with CaO + SrO + BaO + MgO  8-16                                              ZnO 0-5                                                                       Li.sub.2 O 0-2                                                                K.sub.2 O 0-2                                                                 with Li.sub.2 O + Na.sub.2 O + K.sub.2 O 0-2                                ______________________________________                                    

These glasses have a refractive index of more than 1.88, an Abbe numberof more than 29, a density of less than 4.1, and an integratedtransmission from 380 to 800 nm of more than 79% (under a thickness of10 mm).

However, they present two disadvantages:

A very high tendency toward devitrification associated with a very lowliquidus viscosity. The liquidus temperature is on the order of 1150°C.; at this temperature, the viscosity is only a few poise.

A high content of Nb₂ O₅ (20.5-26), which makes the cost of the rawmaterials very high.

Other glasses whose composition presents a certain analogy with that ofthe glasses of the invention are known.

Thus Japanese Patent No. A-3-5340 (1991) describes glasses having arefractive index of 1.83-1.88, whose proportion is, in wt %:

    ______________________________________                                        SiO.sub.2                 2-14                                                  B.sub.2 O.sub.3  8-25                                                         with SiO.sub.2 + B.sub.2 O.sub.3 22-29                                        CaO 13-25                                                                     MgO  0-10                                                                     with CaO + MgO 13-25                                                          La.sub.2 O.sub.3 15-28                                                        BaO + SrO + ZnO  0-12                                                         with La.sub.2 O.sub.3 + BaO + SrO + ZnO 15-28                                 ZrO.sub.2 1-9                                                                 TiO.sub.2  6-15                                                               Nb.sub.2 O.sub.5  6-20                                                        with ZrO.sub.2 + TiO.sub.2 + Nb.sub.2 O.sub.5 24-38                           K.sub.2 O + Na.sub.2 O + Li.sub.2 O 0-3                                     ______________________________________                                    

Although this document mentions glasses that can have a refractive indexof at most 1.88, the highest refractive index of the glasses describedin the example reaches only 1.8725, with most of the glassesspecifically described having much lower indices. Moreover, the CaOcontent of these prior glasses is much higher than that of the glassesof the invention.

German Patent No. C-4,242,859 claims glasses with a high refractiveindex whose composition, defined very broadly, is close to thecomposition of the glasses of the invention, from which it differsnotably by a lower ZrO₂ (<8%) content and by the obligatory presence offluorine. The five specific examples described differ from the glassesof the invention in other compositional differences affecting one ormore constituents.

SUMMARY OF THE INVENTION

The invention concerns glasses having a refractive index of more than1.880, an Abbe number of more than or equal to 30.4 and a density ofless than 4.1, having the following chemical composition expressed in wt% based on the oxides:

    ______________________________________                                        SiO.sub.2               6-9                                                     B.sub.2 O.sub.3 15-19                                                         with SiO.sub.2 + B.sub.2 O.sub.3 <26                                          Nb.sub.2 O.sub.5 12 to <20.5                                                  ZrO.sub.2 8-10                                                                TiO.sub.2 7-11                                                                La.sub.2 O.sub.3 23-32                                                        with ZrO.sub.2 + TiO.sub.2 + La.sub.2 O.sub.3 ≧44                      Y.sub.2 O.sub.3 0-5                                                           WO.sub.3 0-5                                                                  CaO 8-<13                                                                     SrO 0-2                                                                       BaO 0-2                                                                       MgO 0-2                                                                       ZnO 0-5                                                                       with CaO + SrO + BaO + MgO + ZnO ≧9                                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O 0-2                                       Others 0-1                                                                  ______________________________________                                    

Preferred glasses are those having the following chemical compositionexpressed in wt % based on the oxides:

    ______________________________________                                        SiO.sub.2              6.5-8                                                    B.sub.2 O.sub.3 15.5-19                                                       with SiO.sub.2 + B.sub.2 O.sub.3 <26                                          Nb.sub.2 O.sub.5 15-18.5                                                      ZrO.sub.2 ≧8 to 9                                                      TiO.sub.2 9-11                                                                La.sub.2 O.sub.3 25-30                                                        with ZrO.sub.2 + TiO.sub.2 + La.sub.2 O.sub.3 ≧44                      CaO 11 to ≧13                                                          ZnO 0-5                                                                       with CaO + ZnO ≧12                                                     Others 0-1                                                                  ______________________________________                                    

Even more preferred glasses are those having the following chemicalcomposition expressed in wt % based on the oxides:

    ______________________________________                                        SiO.sub.2              6.5-8                                                    B.sub.2 O.sub.3 15.5-19                                                       with SiO.sub.2 + B.sub.2 O.sub.3 <26                                          Nb.sub.2 O.sub.5 15-18.5                                                      ZrO.sub.2 ≧8 to 9                                                      TiO.sub.2 0-10.5                                                              La.sub.2 O.sub.3 25 to <28                                                    with ZrO.sub.2 + TiO.sub.2 + La.sub.2 O.sub.3 ≧44                      CaO 12-<13                                                                    Others 0-1                                                                  ______________________________________                                    

DESCRIPTION OF THE INVENTION

The reduced content of Nb₂ O₅ of these glasses, compared to that ofthose described in European Patent No. B-570,687, allows a significantreduction in the cost of the raw materials.

The glasses of the invention also have an Abbe number at a much higherlevel than those of European Patent No. B-570,687. In fact, Abbe numbersin the 30.4-31.3 range have been obtained, whereas those for the glassesof the examples of European Patent No. B-570,687 are of 29.6-30.6. Thedecrease in the tendency toward devitrification observed for the glassesof the invention is connected with the fact that the principalcrystalline phase that undergoes devitrification in the glasses fromEuropean Patent No. B-570,687 is a complex oxide with a high niobiumcontent. The decrease in the content of this element allows the decreasein the tendency toward devitrification, with the number of crystals andthe rate of their growth being significantly reduced. The most preferredcompositions also present a lower liquidus temperature.

A refractive index of more than 1.88 cannot be obtained if the contentof Nb₂ O₅ is less than 12%.

For the same reason, the contents of ZrO₂, TiO₂, and La₂ O₃ must be atleast 8, 7, and 23%, respectively, and the total of the contents ofthese three elements must be at least 44%.

A ZrO₂ content of more than 10% leads to excessive devitrification.

A La₂ O₃ content of more than 32% leads to an excessively high density.

The color of the glass is very sensitive to the TiO₂ content; for thisreason, this element is limited to 11%.

Although they are less effective than the above-described elements, theoxides of alkaline earth metals and ZnO contribute to the obtention of ahigh index. They present the advantage of having a small influence onthe dispersion. The oxides of alkaline earth metals limit the risk ofdevitrification of zirconium-rich phases. CaO is preferred among theseoxides because it increases the density less than BaO or ZnO. It hasalso been observed that it was more interesting than MgO in regard tothe devitrification. Its level must be between 8 and 13%.

SiO₂ and B₂ O₃ are necessary as glass formation agents. Lower levelsthan 6 and 15%, respectively, lead to an unacceptable devitrification,whereas a total content of these oxides of more than 26% does not allowthe obtention of the desired index.

It is possible to add small quantities of oxides of alkali metals, forexample, to facilitate the melting. However, their total content must belimited to 2%, because these elements decrease the viscosity of theglass and, in particular, the liquidus viscosity. Preferably, they areabsent from the composition.

It is also possible to add the usual refiners, such as As₂ O₃, Sb₂ O₅,fluorides, bromides, or chlorides, provided that their total contentdoes not exceed 1%.

If fairly pure raw materials are used, for example, those contributingan Fe₂ O₃ content of less than 10 ppm, the glass is only weakly colored.If desired, discoloration agents can be added. It is also possible tocolor the glass by adding the usual dyeing elements, for example, oxidesof transition metals or rare earth metals.

The glasses described in the preferred domains ranges are optimized interms of devitrification and density.

The invention is illustrated by the nonlimiting examples given in TableI.

All the proportions are expressed in wt %. Although these examples areonly laboratory experiments, the glasses of the invention can bemanufactured industrially using the classic methods of melting andforming.

In each case, 3000 g of raw materials are melted over 1 h at 1300° C. ina platinum crucible. The glass is then formed in the form of bars havinga thickness of 1 cm. The annealing is carried out at 60° C./h.

The color of the glass was determined by measuring the integratedtransmission, between 380 and 800 nm, of polished samples having athickness of 10 mm.

The tendency toward devitrification was evaluated using the followingtest: for each composition, small pieces of glass placed in platinumcrucibles were retreated for 5 min at 1096, 1112, and 1125° C.,respectively. The crystallization levels observed visually were comparedand a ranking between 1 and 4 was attributed to each glass: 4 for theglasses having the highest tendency to devitrification, and 1 for theglasses possessing the lowest tendency toward devitrification.

Example 1 belongs to the most preferred domain. Examples 2, 3 and 4belong to the preferred domain. Example 8 is not within the scope of theinvention: its Nb₂ O₅ content is too high and it has an unacceptabletendency toward devitrification.

                                      TABLE I                                     __________________________________________________________________________           1   2   3   4   5   6   7   8                                          __________________________________________________________________________    SiO.sub.2                                                                            7.3 7.3 7.3 7.3 7.3 7.3 7.3 +7.3                                         B.sub.2 O.sub.3 16.85 16.65 16.65 17.15 16.65 17.65 18.15 16.65                                                 CaO 12.5 12.5 12.5 12 12 10.5 9 14                                            ZnO       2.5                               TiO.sub.2 9.6 10.1 9.6 9.6 8.6 9.6 9.6 8.6                                    ZrO.sub.2 8.2 8.2 8.5 8.7 8.2 8.2 8.2 8.2                                     Nb.sub.2 O.sub.5 18 15.95 16.9 17.7 18.2 17.7 17.7 21.2                       La.sub.2 O.sub.5 27.5 29.25 28.5 27.5 29 29 27.5 24                           As.sub.2 O.sub.3 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05                      SiO.sub.2 + B.sub.2 O.sub.5 24.15 23.95 23.95 24.45 23.95 24.95 25.45                                          23.95                                        TiO.sub.2 + ZrO.sub.2 + 45.3 47.55 46.6 45.8 45.8 46.8 45.3 40.8                                                La.sub.2 O.sub.3                            CaO + ZnO 12.5 12.5 12.5 12 12 10.5 11.5 14                                   Refractive 1.884 1.885 1.885 1.884 1.883 1.884 1.883 1.883                    Index                                                                         Abbe No. 30.8 31.3 31.3 31 31.3 30.8 30.5 30.4                                Density 4.03 4.06 4.05 4.03 4.07 4.05 4.05 3.98                               Transmission 80.8 80.8 81.1 81.4 81.1 80.5 81.4 80.6                          Tendency 2 1 2 2 2 3 1 4                                                      Toward                                                                        Devitrification                                                               Liquidus (°C.) 1122- 1123- 1143- 1143- 1130- 1152- 1139- 1149-                                            1140 1139 1162 1162 1139 1161 1164                                          1161                                       __________________________________________________________________________

What is claimed is:
 1. Glasses possessing a refractive index of more than 1.880, an Abbe number of at least 30.4, and a density of more than 4.1, and consisting essentially of the following component oxides within the indicated ranges, expressed in wt. %:

    ______________________________________                                         SiO.sub.2               6-9                                                      B.sub.2 O.sub.3 15-19                                                          with SiO.sub.2 + B.sub.2 O.sub.3 <26                                           Nb.sub.2 O.sub.5 12 to <20.5                                                   ZrO.sub.2 8-10                                                                 TiO.sub.2 7-11                                                                 La.sub.2 O.sub.3 23-32                                                         with ZrO.sub.2 + TiO.sub.2 + La.sub.2 O.sub.3 ≧44                       Y.sub.2 O.sub.3 0-5                                                            WO.sub.3 0-5                                                                   CaO 8 to <13                                                                   SrO 0-2                                                                        BaO 0-2                                                                        MgO 0-2                                                                        ZnO 0-5                                                                        with CaO + SrO + BaO + MgO + ZnO ≧9                                     Li.sub.2 O + Na.sub.2 O + K.sub.2 O 0-2                                        Others 0-1.                                                                  ______________________________________                                    


2. Glasses according to claim 1, consisting essentially of the following component oxides within the indicated ranges expressed in wt %:

    ______________________________________                                         SiO.sub.2              6.5-8                                                     B.sub.2 O.sub.3 15.5-19                                                        with SiO.sub.2 + B.sub.2 O.sub.3 <26                                           Nb.sub.2 O.sub.5 15-18.5                                                       ZrO.sub.2 ≧8-9                                                          TiO.sub.2 9-11                                                                 La.sub.2 O.sub.3 25-30                                                         with ZrO.sub.2 + TiO.sub.2 + La.sub.2 O.sub.3 ≧44                       CaO 11 to <13                                                                  ZnO 0-5                                                                        with CaO + ZnO ≧12                                                      Others 0-1                                                                   ______________________________________                                    


3. Glasses according to claim 2, consisting essentially of the following component oxides within the indicated ranges expressed in wt. %:

    ______________________________________                                         SiO.sub.2              6.5-8                                                     B.sub.2 O.sub.3 15.5-19                                                        with SiO.sub.2 + B.sub.2 O.sub.3 <26                                           Nb.sub.2 O.sub.5 15-18.5                                                       ZrO.sub.2 ≧8-9                                                          TiO.sub.2 9-10.5                                                               La.sub.2 O.sub.3 25-<28                                                        with ZrO.sub.2 + TiO.sub.2 + La.sub.2 O.sub.3 ≧44                       CaO 12 to <13                                                                  Others 0-1                                                                   ______________________________________                                    


4. An ophthalmic lens composed of a glass having the properties and composition of claim
 1. 5. An ophthalmic lens composed of a glass having the properties and composition of claim
 2. 6. An ophthalmic lens composed of a glass having the properties and composition of claim
 3. 